Improvements in the technology of physical measurement are often the catalyst for innovation. For example, modern automobile engines incorporate a measurement system to continuously monitor the performance of an engine during operation. The measurements are then used to actively tune the engine during operation. The continuous cycle of measuring the performance and tuning the engine has increased the fuel efficiency and power of the engine while also decreasing the pollutants emitted by the engine.
The need for improved sensors for physical measurement is widely recognized. One type of conventional sensor is the spring-activated sensor in which a spring provides a biasing force. Spring-activated sensors may be used in several applications, such as pressure gauges and supermarket weight scales. The spring-activated sensor operates by balancing a load against the biasing force and determining the amount of deflection in the spring. Spring-activated sensors have several disadvantages. For example, spring-activated sensors generally do not have a high degree of accuracy and must often be recalibrated due to changes in the physical properties of the spring. In addition, spring-activated sensors do not operate reliably at high temperatures and can have slow response times.
More accurate sensors have been provided by using piezoelectric transducers as sensors in such applications as pressure gauges and scales. Piezoelectric transducers incorporate a piezoelectric crystal that produces an electrical signal in response to distortion of the crystal structure. The greater the distortion of the crystal structure, the greater the electrical signal produced by the crystal. Piezoelectric transducers also have several disadvantages. For example, piezoelectric transducers do not operate in high temperature environments and must be recalibrated frequently. In addition, the operating life of the piezoelectric transducer may be relatively short and the sensors are relatively expensive. Furthermore, the piezoelectric transducer is not well suited for extremely accurate measurements. Other conventional measurement devices, such as strain gauges, have similar disadvantages.
The optical fiber has proven to be a versatile and relatively efficient means of transporting light energy and information. For example, optical fibers are used in the medical field to transport laser energy through flexible catheters for pin-point microsurgery, or in the telecommunications field to transport data over long distances at very high rates. Recent developments in optical fiber technology allow very accurate measurement of a small change in the length of a portion of the optical fiber.
Commonly assigned U.S. Pat. No. 5,452,087 describes one technique for measuring pressure with embedded nonintrusive fiber optics. This patent describes the use of a fiber Fabry-Perot Interferometer element in an optic cable that is embedded into a metal part that is then fastened into a larger structure, such as a pressure vessel wall. The embedded construction of the sensor has several disadvantages. For example, the embedded sensor must be fastened into a larger structure. In addition, the embedded sensor does not readily lend itself to measurements other than strain or pressure in a vessel.
Similarly, commonly assigned U.S. Pat. No. 5,714,680 describes an embedded fiber optic sensor for measuring pressure. In that patent, the fiber Fabry-Perot interferometer element is embedded into a metal part that is located in a housing. Pressure acting on one end of the metal part compresses the metal part which is sensed by the fiber Fabry-Perot interferometer element, thereby providing a measurement of the pressure acting on the metal part. This embedded sensor suffers from many of the same disadvantages as the embedded sensor discussed above.